I. Field
The following description relates generally to communication systems, and more particularly, to a method and apparatus for clock drift compensation during acquisition in a wireless communication system.
II. Background
In order for a wireless device to receive a transmission, which includes a plurality of packets, the receiver of the wireless device must perform an acquisition operation to acquire the received signal. Each packet begins with a preamble, which generally includes a known sequence. The acquisition process includes packet detection, symbol timing estimation, carrier frequency offset estimation and carrier phase estimation by means of the preamble so that the information following in the rest of the packet can be correctly demodulated.
Prior to the reception and data demodulation of a packet that is transmitted using a waveform; the receiver needs to determine where the beginning of the packet occurs in the waveform. The acquisition process usually consists of at least two steps, with the first step being sampling the waveform to determine where in the packet the receiver has started receiving the waveform, and the second step is performing hypothesis testing. The hypothesis testing refers to the computation of the correlation between the known sequence and the demodulated sequences starting at various positions. The signal is considered to be acquired when the largest correlation value is larger than a pre-defined threshold. The starting position of the hypothesis is the time base on where all the remaining operations are based. Typically, a tracking process follows the acquisition to further refine the timing, but the time base needs to be acquired with relative accuracy.
During the period of time needed for the acquisition process to complete, clock drift may occur between the transmitter and the receiver. Had there been no clock drift between the transmitter and receiver, the time base obtained at acquisition stage could be used for tracking and other operations without error. Even when clock drift exists, it is not a significant problem in most existing communication systems. This is due to either highly accurate oscillators (CDMA2000 EV-DO requires clock drift within ±0.5 parts per million (ppm)), and/or narrow signal bandwidth (IEEE 802.11a signal uses 20 MHz bandwidth, the clock drift is required to be within ±20 ppm). However, the oscillators found in low-cost devices are usually not very accurate. For example, if the transmitter and receiver use oscillators that drift in the range of ±100 ppm, which equals a maximum of 200 ppm between the transmitting and receiving devices. Thus, if a sampling phase completes in 12.8 microseconds (μs) and a computation phase completes in 10 μs. In the worst case, the clock has drifted 4.6 nanoseconds (ns) by the time the receiver finishes the acquisition process. The time base thus derived may thus be off by as large as 3 ns even if error that is due to noise is ignored. The factors that cause degradation of acquisition performance include noise, interference, and clock drift between the two ends of the communication channel. For low-cost pulse-based ultra-wideband devices, clock drift can be extremely detrimental. For example, a 2-ns offset can sometime cause the receiver to lose most of the pulses and lead to acquisition failure. In a pulse-based ultra-wideband (UWB) system, this could be a significant portion of the pulse. Without proper compensation for the drift, the completion of the acquisition is likely to fail.
Consequently, it would be desirable to address one or more of the deficiencies described above.